Line Graph
A line graph plots measured data points and connects them to show how a dependent variable changes as the independent variable changes.
Definition
A line graph shows how something changes: you plot dots for your measurements and connect them so the pattern can be seen going up, down, or staying flat. The independent variable goes on the x-axis and the dependent variable on the y-axis, each point usually marks a mean of trials, and the graph is used when the independent variable is continuous, so values between the points are meaningful. In lab science each point carries error bars showing the range or standard deviation, and the line is either a best-fit curve from a model or a point-to-point connection when no model is assumed. Scale choices, such as where the axis starts, shape what the eye reads and must be stated.
Example
Plotting days along the bottom and height up the side, points at $(1, 2.0)$, $(2, 2.5)$, $(3, 3.1)$, and $(4, 3.8)$ show a bean plant growing a little faster each day. Salt-dissolving time against temperature, with points at $(10^\circ\text{C}, 48\ \text{s})$, $(30^\circ\text{C}, 26\ \text{s})$, and $(50^\circ\text{C}, 14\ \text{s})$, gives a falling line whose steeper drop at the low end hints that the relationship is not perfectly straight. For cart speed versus ramp angle, a group overlays the curve $v = \sqrt{2gL\sin\theta}$ for their $1.5\ \text{m}$ ramp instead of connecting dots and finds every measured point about $8\%$ below it, a systematic gap they attribute to friction.
Key Insight
Use a line graph when the bottom axis is continuous, like time or temperature; use a bar graph to compare separate categories. The line is a claim about what happens between your measurements, so connect the dots only if you believe the variable changes smoothly between them. A line graph tests a model as much as it shows data: when measured points bend away from the predicted line in the same direction every time, that is evidence about the physics, not noise.